High conductivity copper alloy for semi-solid forming and manufacturing method thereof

ABSTRACT

Disclosed is a copper alloy having high electrical conductivity of at least 70% IACS and a broad solid-liquid two-phase region, capable of being easily subjected to a semi-solid forming process even at various temperatures. The copper alloy is suitable for use in a rotor of small- and medium-sized electric motors. In addition, the prevent invention provides a method of manufacturing the copper alloy for semi-solid forming, including the step of melting a copper-calcium alloy having 0.1-1.5 wt % of calcium and the balance of copper at 1100-1150° C. to form a molten copper-calcium alloy, which is then maintained at 1100-1150° C. for a predetermined time period. Then, the molten copper-calcium alloy is poured into a mold preheated to 100-150° C. and cast to a copper alloy ingot. Thereafter, in order to remove segregation and stress generated upon casting and to convert a dendritic strucutre of a primary copper phase to a globular structure, the copper alloy ingot is subjected to thermomechanical treatment, thereby obtaining the primary copper phase comprising the globular structure suitable for semi-solid forming.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention pertains to a high conductivity copper alloy for semi-solid forming and manufacturing methods thereof. More specifically, the present invention pertains to a copper alloy having higher electrical conductivity and broader solid-liquid two-phase region compared to conventional copper alloys including Cu-0.11% O alloy, which can be easily processed by semi-solid forming to produce compact, energy efficient electric motors; and a method of manufacturing the same.

[0003] 2. Description of the Prior Art

[0004] Electric motor is an apparatus to covert electrical energy to mechanical work, during which approximately 7 to 25% of energy losses occur. Currently, more than 50% of the electricity is consumed by small and medium sized motors in the world. Based on industrial development and economic growth, future power consumption rate is expected to gradually increase. Particularly, in Korea, more than two times greater electric power compared to that of 1995 is expected to be required in 2005.

[0005] According to the U.S. Department of Energy (DOE), electric motors larger than ⅙ hp are reported to consume 60% of the total electric power produced in United States, in which 60% of the electric power consumed by the electric motors is consumed by small and medium sized motors of 1-25 hp.

[0006] Thus, improving the efficiency of the motors is closely related to energy savings. An efficiency of the electric motor is represented by a ratio of output mechanical energy to input electric energy. The higher the efficiency of the electric motor, the smaller the electric energy consumed. The efficiency of induction motors is significantly influenced by the electrical conductivity of a rotor within the motors.

[0007] Commercial purity aluminum has been widely used as the material for the rotor of small and medium sized motors. Pure aluminum (Al) has an electrical conductivity corresponding to about 60% of that of pure copper (Cu), and energy loss thereof is high due to large electric resistance. Thus, replacing the aluminum rotor by a high conductivity copper alloy can improve the efficiency of the induction motors. However, the use of copper alloys has been limited only to the rotor of large-sized motors in current, produced by expensive labor-intensive works.

[0008] An aluminum rotor for medium- and small-sized motors has been produced by die-casting process. This is because aluminum is inexpensive and easy to form into complicated shapes. In addition, the aluminum rotor can be manufactured using low cost steel molds because of lower melting point.

[0009] However, the copper rotor is difficult to produce by die-casting process because expensive refractory materials should be used for the mold due to high melting point of pure copper (1085° C.), increasing the production cost.

[0010] Different from the die-casting, semi-solid forming process is carried out in a solid-liquid two-phase region. Therefore, the operating temperature for the semi-sold forming process is 100-200° C. lower than that for die-casting process, reducing energy consumption and increasing die life.

[0011] The alloy for semi-solid forming should have sufficient range of melting. If the solid-liquid two-phase region is too narrow, the alloy is easily melted or solidified at small temperature variations, making it difficult to conduct semi-solid processing.

[0012] Another factor to consider for the rotor material is that the alloy should have a reasonable electrical conductivity, higher conductivity than that of pure aluminum. Practically, the copper alloy requires to have an electrical conductivity of at least 70% IACS. However, the addition of alloying elements to pure copper reduces the electrical conductivity, although the reduction rate depends on alloying elements.

[0013] The research on the development of copper alloy for semi-solid forming has not been intensively conducted. In particular, few attempts to develop high electrical conductivity copper alloys for semi-solid forming have been carried out. Semi-solid forming processing for the rotor has been attempted on a laboratory scale using oxygen-copper (Cu-0.11% O), which has, however, relatively narrow range of melting, 17 to 20° C., and thus is difficult to practically use.

[0014] An object of present invention is to provide a copper alloy for semi-solid forming, having a sufficiently broad solid-liquid two-phase region and high electrical conductivity of at least 70% IACS, whereby the copper alloy can be easily processed by semi-solid forming and can be applied to manufacture a rotor of small- and medium-sized electric motors. Another object of the present invention is to provide a method of manufacturing the high conductivity copper alloy for the semi-solid forming.

SUMMARY OF THE INVENTION

[0015] According to the present invention, 0.1-1.5 wt % of calcium is added to copper to form a copper-calcium alloy, which is then heated, thereby obtaining a primary phase of the alloy comprising a globular structure. Such an alloy comprising the globular structure can be subjected to a semi-solid forming process at a broader solid-liquid two-phase region covering a temperature range of 130° C. or more, with electrical conductivity of at least 75% IACS, thereby considerably improving the efficiency of a power-saving in small- and medium-sized electric motor.

[0016] To achieve the above objects, the present invention provides a high conductivity copper alloy for semi-solid forming, comprising 0.1-1.5 wt % of calcium and the balance of copper cast together, to form a cast calcium-copper alloy.

[0017] As for the copper alloy, the cast calcium-copper alloy is subjected to heat treatment to have an electrical conductivity of 75-95% IACS and a solid-liquid two-phase region exhibiting a temperature range of 910 to 1085° C.

[0018] As for the copper alloy, the cast calcium-copper alloy is compressed by 0.1-30% before heat treatment.

[0019] In addition, the present invention provides a method of manufacturing a high conductivity copper alloy for semisolid forming, comprising the steps of melting a copper-calcium alloy including 0.1-1.5 wt % of calcium and the balance of copper at 1100-1150° C. to form a molten copper-calcium alloy, which is then maintained at the same temperature for 1-5 min, casting the molten copper-calcium alloy to a copper alloy ingot by use of a mold preheated to 100-150° C., and heating the copper alloy ingot to 910-1085° C. corresponding to a solid-liquid two-phase region and maintaining the heated copper alloy ingot at 910-1085° C. for 5-10 min, followed by chilling the heated ingot to remove segregation and stress generated in the casting step and to convert a dendritic strucutre of a primary copper phase to a globular structure, thereby obtaining the primary copper phase comprising the globular structure suitable for semi-solid forming.

[0020] The manufacturing method of the copper alloy further comprises the step of compressing the copper alloy ingot by 0.1-30%, before the heating step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a diagram illustrating a reduction of electrical conductivity according to increase of a content of calcium in a copper-calcium alloy;

[0022]FIG. 2 is a diagram illustrating thermal analysis results of the copper-calcium alloy, showing solid-liquid two-phase regions and eutectic peaks (912° C.), enlarging with increasing the content of calcium in the alloy;

[0023]FIG. 3a is an optical microphotograph showing copper dendrites and Cu—Cu₅Ca eutectic structure of as-cast Cu-0.22Ca alloy;

[0024]FIG. 3b is an optical microphotograph showing a globular primary copper in Cu-0.22Ca alloy after being subjected to compression and heat treatment for, semi-solid forming;

[0025]FIG. 3c is an optical microphotograph showing a globular primary copper in Cu-0.22Ca alloy after being subjected to only heat treatment for semi-solid forming;

[0026]FIG. 4a is an optical microphotograph showing copper dendrites and Cu—Cu₅Ca eutectic structure of as-cast Cu-0.34Ca alloy;

[0027]FIG. 4b is an optical microphotograph showing a globular primary copper in Cu-0.34Ca alloy after being subjected to compression and heat treatment for semi-solid forming;

[0028]FIG. 4c is an optical microphotograph showing a globular primary copper in Cu-0.34Ca alloy after being subjected to only heat treatment for semi-solid forming;

[0029]FIG. 5a is an optical microphotograph showing copper dendrites and Cu—Cu₅Ca eutectic structure of as-cast Cu-0.69Ca alloy;

[0030]FIG. 5b is an optical microphotograph showing a globular primary copper in Cu-0.69Ca alloy after being subjected to compression and heat treatment for semi-solid forming;

[0031]FIG. 5c is an optical microphotograph showing a globular primary copper in Cu-0.69Ca alloy after being subjected to only heat treatment for semi-solid forming; and

[0032]FIG. 6 is a diagram illustrating a liquid fraction ratio of the copper-calcium alloy, in which the liquid fraction ratio increases with increasing calcium content.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Hereinafter, a detailed description will be given of a high conductivity copper alloy for semi-solid forming and a manufacturing method thereof, in connection with the attached drawings.

[0034] As for the copper alloy suitable for use in semi-solid forming of the present invention, inexpensive calcium is added to pure copper to broaden a solid-liquid two-phase region. Thereby, it is possible to perform a semi-solid forming process. In addition, an electrical conductivity of the copper alloy retains at least 75% IACS, whereby the copper alloy can be used for preparation of a rotor of an electric motor.

[0035] The copper-calcium alloy of the present invention is characterized by having a solid-liquid two-phase region covering a temperature range of 150° C. or more, with high electrical conductivity of at least 80% IACS when 1 wt % or less of calcium is contained in the alloy. In addition, when the copper alloy is subjected to compression and heat treatment for semi-solid forming, all dendritic structures of a primary copper phase are converted to a globular structure. Thereby, the copper alloy can be easily subjected to semi-solid forming.

[0036] In the present invention, the copper alloy suitable for use in the semi-solid forming comprises 0.1-1.5 wt % of calcium, the balance being copper.

[0037] A calcium component (Ca) reacts with copper (Cu) to produce a Cu₅Ca intermetallic compound having a melting point of 950° C. A eutectic reaction between copper and Cu₅Ca intermetallic compound comprising Cu-7 wt % Ca occurs at 917° C. Hence, the solid-liquid two-phase region of the-copper alloy covers a temperature range of up to 168° C. As the content of calcium in the copper alloy increases, the temperature range of the solid-liquid two-phase region decreases.

[0038] The electrical conductivity of copper is reduced with increasing the content of alloying elements, in which the reduction rate depends on the type of alloying elements.

[0039] The average reduction rate of electrical conductivity of the copper-calcium alloy of present invention was 22.1% IACS/wt % Ca. Upon the addition of 1.1 wt % of calcium to copper, the inventive alloy has high electrical conductivity of about 77% IACS.

[0040] Based on the present invention, a copper alloy suitable for use in a semi-solid forming process should have high electrical conductivity of at least 70% IACS and a broad solid-liquid two-phase region.

[0041]FIG. 1 shows the variations in electrical conductivity of copper on the calcium content in as-cast specimens. The specimens were prepared by melting the copper-calcium alloy in an induction furnace at 1120° C., followed by pouring the molten copper alloys into a mold preheated to 120° C., thereby yielding a cast copper alloy ingot.

[0042] As shown in FIG. 1, according to ICP analytic composition of the sample, when the calcium component is used in the amount of 0.052, 0.22, 0.34, 0.69 and 1.07 wt %, electrical conductivity of the copper-calcium alloy is gradually reduced in the range of from 100% IACS to 78% IACS. In such a case, the average reduction rate of electrical conductivity on the content of calcium in the copper alloy is calculated to be 22.1% IACS/wt % Ca.

[0043] This means that electrical conductivity of copper is not drastically decreased with increasing calcium content. Even though large amount of calcium up to 1.1 wt % is used, the inventive copper alloy showed relatively high electrical conductivity.

[0044] In addition to the high electrical conductivity, the copper alloy for semi-solid forming should have a reasonable range of solid-liquid two-phase region. In copper-calcium alloy, the eutectic reaction between copper and the Cu₅Ca intermetallic compound occurs at 917° C. in an equilibrium phase diagram, in which there exists a two-phase region in the composition range of from ˜0 Ca to 7 wt % Ca in Cu—Ca. Such a two-phase region covers a range of temperatures of 168° C. in the equilibrium phase diagram. As the content of calcium increases, the temperature range of the two-phase region decreases.

[0045] The temperature range of the solid-liquid two-phase region of a compatible alloy is confirmed by thermal analysis. FIG. 2 shows the thermal analysis results by DTA (Differential Thermal Analyzer) for each alloy heated from room temperature to 1100° C. As shown in FIG. 2, all samples showed a strong endothermic reaction in the temperature range of 910-920° C. during heating. The energy absorption peaks are enlarged in proportion to the increase of the content of calcium. After completion of the primary endothermic reaction, a second strong endothermic reaction occurred at 1070° C. or higher. The endothermic reaction in a DTA curve means that a part of any phase in the alloy sample is melted. The primary endothermic peak corresponds to a eutectic reaction temperature of Cu—Ca system. The second endothermic reaction occurs while the primary copper phase cast to a dendritic structure is melted.

[0046] From the above results, it can be found that the copper-calcium alloy of the present invention has a solid-liquid two-phase region covering a temperature range of 150° C. or more.

[0047] The microstructure of primary phase should be in globular shape for the materials to be easily formed by semi-solid processing because dendrite structure hinders uniform materials flow during thixoforming and promotes solid-liquid phase separation. In the present invention, with the aim of converting the dendritic structure of the copper primary phase in copper-calcium alloy to the globular structure, compression and heat treatment are performed.

[0048] The copper-calcium alloy samples without compression, and the alloy samples compressed by 13%, 20% and 33% were induction-heated to 1050° C. corresponding to a solid-liquid two-phase region and maintained at the same temperature for 7 min. FIGS. 3a, 4 a, and 5 a are optical microphotographs of the cast copper-calcium alloys. FIGS. 3b, 4 b, and 5 b are optical microphotographs of the copper-calcium alloys after compression and heat treatment. FIGS. 3c, 4 c, and 5 c are optical microphotographs of the copper-calcium alloy after only heat treatment.

[0049] As shown in FIGS. 3a, 4 a, and 5 a, typical dendritic strucutres are observed in the as-cast samples. The dendritic structure of the primary copper phase was changed to globular structures that are suitable for semi-solid forming process after compression and heat treatment, as shown in FIGS. 3b, 4 b, and 5 b. In the globular primary copper phase, twin crystals were also observed, which are formed during compressing and heating.

[0050] In the samples that are subjected to heat treatment without compression (compression rate 0%), the primary copper phase showed a globular structure completely converted from the dendritic structure as shown in FIGS. 3c, 4 c and 5 c. Hence, it can be confirmed that a uniform structure of the copper alloy suitable for semi-solid forming can be obtained even though only a heating process is performed.

[0051]FIG. 6 illustrates a liquid fraction ratio of the copper-calcium alloy ranging from 10 to 20%, obtained by image analysis. As shown in FIG. 6, the larger the content of calcium in copper, the higher the liquid fraction ratio.

[0052] As described above, the present invention provides a copper alloy for semi-solid forming, characterized by having an electrical conductivity of 75-95% IACS, with a solid-liquid two-phase region covering a temperature range of 150° C. or more. Thus, the inventive copper alloy is suitable for semi-solid forming process at temperatures 100-200° C. lower than that of a die-casting process.

[0053] Compared to conventional oxygen-copper (Cu-0.11% O) alloys, the inventive copper alloy has a solid-liquid two-phase region covering a temperature range broadened by 130° C. or more. Thus, the inventive alloy is not influenced by temperature variations during the processing, and has electrical conductivity suitable for use in a rotor of an electric motor.

[0054] In order to convert a dendritic structure to a globular structure suitable for semi-solid forming, the inventive alloy can be processed only by heat treatment without prior deformation, thereby reducing energy consumption and the number of processes, compared to a semi-solid forming alloy requiring a compression process.

[0055] At present, about 50% or more of total global electric power is consumed by electric motors. Accordingly, when a rotor of the small- and medium-sized motors is made of the copper alloy of the present invention, the motor efficiency, combined with enhanced designs, is expected to increase by 1-1.5%.

[0056] The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. 

What is claimed is:
 1. A high conductivity copper alloy for semi-solid forming, comprising 0.1-1.5 wt % of calcium and the balance of copper cast together, to form cast copper-calcium alloys.
 2. The copper alloy as defined in claim 1, wherein the cast copper-calcium alloy is subjected to heat treatment to have a solid-liquid two-phase region exhibiting a temperature range of 910 to 1085° C. and an electrical conductivity of 75-95% IACS.
 3. The copper alloy as defined in claim 1 or 2, wherein the cast copper-calcium alloy is compressed by 0.1-30% before heat treatment.
 4. A method of manufacturing a high conductivity copper alloy for semi-solid forming, comprising the following steps of: melting a copper-calcium alloy including 0.1-1.5 wt % of calcium and the balance of copper at 1100-1150° C. to form molten copper-calcium alloys, which is then maintained at the same temperature for 1-5 min; casting the molten copper-calcium alloy to a copper alloy ingot by use of a mold preheated to 100-150° C.; and heating the copper alloy ingot to 910-1085° C. corresponding to a solid-liquid two-phase region and maintaining the heated copper alloy ingot at 910-1085° C. for 5-10 min, followed by chilling the heated copper alloy ingot, to remove segregation and stress generated in the casting step and to convert a dendritic structure of a primary copper phase to a globular structure, thereby obtaining the primary copper phase comprising the globular structure suitable for semi-solid forming.
 5. The method as defined in claim 4, further comprising the step of compressing the copper alloy ingot by 0.1-30%, before the heating step. 